Speciated isotope dilution analysis of Cr(III) and Cr(VI) in water by ICP-DRC-MS

An isotope dilution method has been developed for the speciation analysis of chromium in natural waters which accounts for species interconversions without the requirement of a separation instrument connected to the mass spectrometer. The method involves (i) in-situ spiking of the sample with isotopically enriched chromium species; (ii) separation of chromium species by precipitation with iron hydroxide; (iii) careful measurement of isotope ratios using an inductively coupled plasma mass spectrometer (ICP-MS) with a dynamic reaction cell (DRC) to remove isobaric polyatomic interferences. The method detection limits are 0.4 μg L⁻¹ for Cr(III) and 0.04 μg L⁻¹ for Cr(VI). The method is demonstrated for the speciation of Cr(III) and Cr(VI) in local nullah and synthetically spiked water samples. The percentage of conversion from Cr(III) to Cr(VI) increased from 5.9% to 9.3% with increase of the concentration of Cr(VI) and Cr(III) from 1 to 100 μg L⁻¹, while the reverse conversion from Cr(VI) to Cr(III) was observed within a range between 0.9% and 1.9%. The equilibrium constant for the conversion was found to be independent of the initial concentrations of Cr(III) and Cr(VI) and in the range of 1.0 (at pH 3) to 1.8 (at pH 10). The precision of the method is better than that of the DPC method for Cr(VI) analysis, with the added bonuses of freedom from interferences and simultaneous Cr(III) determination.     

A novel flow injection chemiluminescence determination of Cr(VI) with Dichlorotris(1,10-phenanthroline)ruthenium(II)

A selective novel reverse flow injection system with chemiluminescence detection (rFI-CL) for the determination of Cr(VI) in presence of Cr(III) with Dichlorotris (1,10-phenanthroline)ruthenium(II), (Ru(phen)₃Cl₂), is described in this work. This new method is based on the oxidation capacity of Cr(VI) in H₂SO₄ media. First, the Ruthenium(II) complex is oxidized to Ruthenium(III) complex by Cr(VI) and afterwards it is reduced to the excited state of the Ruthenium(II) complex by a sodium oxalate solution, emitting light inside the detector. The intensity of chemiluminescence (CL) is proportional to the concentration of Cr(VI) and, under optimum conditions, it can be determined over the range of 3-300 μg L⁻¹ with a detection limit of 0.9 μg L⁻¹. The RSD was 8.4% and 1.5% at 5 and 50 μg L⁻¹, respectively. For the rFI-CL method various analytical parameters were optimized: flow rate (1 mL min⁻¹), H₂SO₄ carrier concentration (20% w/V), Ru(phen)₃Cl₂ concentration (5 mM) and sodium oxalate concentration (0.1 M). The effect of Cr(III), Fe(III), Al(III), Cd(II), Zn(II), Hg(II), Pb(II), Ca(II) and Mg(II), was studied. The method is highly sensitive and selective, allowing a fast, on-line determination of Cr(VI) in the presence of Cr(III). Finally, the method was tested in four different water samples (tap, reservoir, well and mineral), with good recovery percentage.     

Hybrid mesoporous materials for on-line preconcentration of Cr(VI) followed by one-step scheme for elution and colorimetric determination at ultratrace levels

An hybrid mesoporous material synthesised in our laboratories for solid phase extraction (SPE) in flow through systems has been used for analytical purposes. The solid was obtained from mesoporous silica MCM-41 functionalized with 3-aminopropyltriethoxy silane by Sol-Gel methodology. In order to exploit the large sorption capacity of the material together with the possibility of modeling it for anions retention, a microcolumn (MC) filled with the solid was inserted in a flow system for preconcentration of Cr(VI) and its determination at ultratrace levels in natural waters. The analytical methodology involved a reverse flow injection system (rFI) holding a MC filled with the solid for the analyte extraction. Elution and colorimetric detection were carried out with 1-5 diphenylcarbazide (DPC) in sulfuric acid. DPC produced the reduction of Cr(VI) to Cr(III) together with the generation of a cationic red complex between Cr(III) and 1-5 diphenylcarbazone which was easily eluted and detected with a visible spectrophotometer. Moreover, the filling material got ready for the next sample loading remaining unspoiled for more than 300 cycles.The effect of several variables on the analytical signal as well as the influence of cationic and anionic interferences were discussed. Particular attention was given to sulfuric acid interference since it is the required media for the complex generation.Under optimal conditions, 99.8% of Cr(VI) recovery was obtained for a preconcentration time of 120 s (sample and DPC flow rates = 1 mL min⁻¹) and an elution volume of 250 μL. The limit of detection (3 s) was found to be 0.09 μg L⁻¹ Cr(VI) with a relative standard deviation (n = 10, 3 μg L⁻¹) of 1.8.Since no Cr(III) was retained by the solid material and Cr(VI) was completely adsorbed, electrothermal atomic absorption spectrometry (ET AAS) determinations of Cr(III) were also performed by simply measuring its concentration at the end of the microcolumn after Cr(VI) retention by the mesoporous solid.Applications to the determination of Cr(VI) and Cr(III) in natural waters and the validation of the methodology were also studied.     

Sorptive potential of sunflower stem for Cr(III) ions from aqueous solutions and its kinetic and thermodynamic profile

The sorptive potential of sunflower stem (180-300 μm) for Cr(III) ions has been investigated in detail. The maximum sorption (≥85%) of Cr(III) ions (70.2 μM) has been accomplished using 30 mg of high density sunflower stem in 10 min from 0.001 M nitric and 0.0001 M hydrochloric acid solutions. The accumulation of Cr(III) ions on the sorbent follows Dubinin-Radushkevich (D-R), Freundlich and Langmuir isotherms. The isotherm yields D-R saturation capacity X_m = 1.60 ± 0.23 mmol g⁻¹, β = −0.00654 ± 0.00017 kJ² mol⁻², mean free energy E = 8.74 ± 0.12 kJ mol⁻¹, Freundlich sorption capacity K_F = 0.24 ± 0.11 mol g⁻¹, 1/n = 0.90 ± 0.04 and of Langmuir constant K_L = 6800 ± 600 dm³ mol⁻¹ and C_m = 120 ± 18 μmol g⁻¹. The variation of sorption with temperature (283-323 K) gives ΔH = −23.3 ± 0.8 kJ mol⁻¹, ΔS = −64.0 ± 2.7 J mol⁻¹ K⁻¹ and ΔG_298k = −4.04 ± 0.09 kJ mol⁻¹. The negative enthalpy and free energy envisage exothermic and spontaneous nature of sorption, respectively. Bisulphate, Fe(III), molybdate, citrate, Fe(II), Y(III) suppress the sorption significantly. The selectivity studies indicate that Cr(III), Eu(III) and Tb(III) ions can be separated from Tc(VII) and I(I). Sunflower stem can be used for the preconcentration and removal of Cr(III) ions from aqueous medium. This cheaper and novel sorbent has potential applications in analytical and environmental chemistry, in water decontamination, industrial waste treatment and in pollution abatement. A possible mechanism of biosorption of Cr(III) ions onto the sunflower stem has been proposed.     

Speciation of Cr(III) and Cr(VI) in waters using immobilized moss and determination by ICP-MS and FAAS

The possibility of using moss (Funaria hygrometrica), immobilized in a polysilicate matrix as substrate for speciation of Cr(III) and Cr(VI) in various water samples has been investigated. Experiments were performed to optimize conditions such as pH, amount of sorbent and flow rate, to achieve the quantitative separation of Cr(III) and Cr(VI). During all the steps of the separation process, Cr(III) was selectively sorbed on the column of immobilized moss in the pH range of 4-8 while, Cr(VI) was found to remain in solution. The retained Cr(III) was subsequently eluted with 10 ml of 2 mol l⁻¹ HNO₃. A pre-concentration factor of about 20 was achieved for Cr(III) when, 200 ml of water was passed. The immobilized moss was packed in a home made mini-column and incorporated in flow injection system for obtaining calibration plots for both Cr(III) and Cr(VI) at low ppb levels that were compared with the plots obtained without column. After separation, the chromium (Cr) species were determined by inductively coupled plasma mass spectrometry (ICP-MS) and flame atomic absorption spectrometry (FAAS). The sorption capacity of the immobilized moss was found to be ∼11.5 mg g⁻¹ for Cr(III). The effect of various interfering ions has also been studied. The proposed method was applied successfully for the determination of Cr(III) and Cr(VI) in spiked and real wastewater samples and recoveries were found to be >95%.     

Flow injection solid phase extraction electrothermal atomic absorption spectrometry for the determination of Cr(VI) by selective separation and preconcentration on a lab-made hybrid mesoporous solid microcolumn

A lab-made hybrid mesoporous solid was employed in a flow injection solid phase extraction electrothermal atomic absorption spectrometric (FI–SPE–ETAAS) system for the selective retention of Cr(VI). The solid was prepared by co-condensation of sodium tetraethylortosilicate and 3-aminopropyltriethoxysilane by sol–gel methodology and one-pot synthesis and characterized by Fourier transform infrared spectroscopy, X ray diffraction spectroscopy, and scanning electronic microscopy. Adsorption capacities at different pH values of both, Cr(VI) and Cr(III), were also measured in order to obtain the optimum retention for Cr(VI) with no interference of Cr(III). The maximum capacity of adsorption (4.35 mmol g^− 1) was observed for pH values between 2–3, whilst Cr(III) was found to remain in solution (adsorption capacity = 0.007 mmol g^− 1). Then, a microcolumn (bed volume: 7.9 µL) was filled with the solid and inserted in the FI–ETAAS system for analytical purposes. Since the analyte was strongly retained by the filling in the anionic form, 0.1 mol L^− 1 hydroxylammonium chloride in 1 mol L^− 1 hydrochloric acid was selected as eluent due to its redox characteristics. In this way, the sorbed Cr(VI) was easily released in the cationic form. The enrichment factor (EF) was found as a compromise between sensitivity and sample throughput and a value of 27 was obtained under optimized conditions: pH 2, sample loading 2 mL min^− 1 (60 s), elution flow rate 0.5 ml min^− 1 (eluent volume: 75 μL).     

Chemically modified silica gel with p-dimethylaminobenzaldehyde for selective solid-phase extraction and preconcentration of Cr(III), Cu(II), Ni(II), Pb(II) and Zn(II) by ICP-OES

Silica gel-bound amines phase modified with p-dimethylaminobenzaldehyde (p-DMABD) was prepared based on chemical immobilization technique. The product (SG-p-DMABD) was used as an adsorbent for the solid-phase extraction (SPE) Cr(III), Cu(II), Ni(II), Pb(II) and Zn(II) prior to their determination by inductively coupled plasma optical emission spectrometry (ICP-OES). The uptake behaviors of SG-p-DMABD for extracting these metal ions were studied using batch and column procedures. For the batch method, the optimum pH range for Cr(III) and Ni(II) extraction was ≥ 3, for Cu(II), Pb(II) and Zn(II) extraction it was ≥ 4. For simultaneous enrichment and determination of all the metals on the newly designed adsorbent, the pH value if 4.0 was selected. All the metal ions can be desorbed with 2.0 mL of 0.5 mol L^− 1 of HCl. The results indicate that SG-p-DMABD has rapid adsorption kinetics using the batch method. The adsorption capacity for these metal ions is in the range of 0.40-1.15 mmol g^− 1, with a high enrichment factor of 125. The presence of commonly coexisting ions does not affect the sorption capacities. The detection limits of the method were found to be 1.10, 0.69, 0.99, 1.10 and 6.50 μg L^− 1 for Cr(III), Cu(II), Ni(II), Pb(II) and Zn(II), respectively. The relative standard deviation (RSD) of the method under optimum conditions was 5.0% (n = 8) for all metal ions. The method was applied to the preconcentration of Cr(III), Cu(II), Ni(II), Pb(II) and Zn(II) from the certified reference material (GBW 08301, river sediment) and water samples with satisfactory results.     

Simultaneous, sensitive and selective on-line chemiluminescence determination of Cr(III) and Cr(VI) by capillary electrophoresis

A novel in-capillary reduction and capillary electrophoretic (CE)-chemiluminescence (CL) method was developed for the sensitive and selective determination of chromium(III) and chromium(VI). The proposed method was based on the in-capillary reduction of Cr(VI) with acidic H₂O₂ to form Cr(III) using the zone-passing technique and chemiluminescence detection of Cr(III). The sample [Cr³⁺ and CrO₄²⁻], hydrochloric acid, and H₂O₂ (reductant) solution segments were injected for specified periods of time in this order from the anodic end of a capillary, followed by application of an appropriate running voltage between both ends. As both chromium species have opposite charges, Cr³⁺ migrates to the cathode while CrO₄²⁻ ion, moving oppositely to the anode, reacts with acidic H₂O_2, resulted in formation of Cr³⁺. Based on the migration time difference of both Cr³⁺ ions, they were separated by zone electrophoresis. Running buffer was composed of 0.02 mol l⁻¹ HAc-NaAc (pH 4.7) with 1×10⁻³ mol l⁻¹ EDTA. Parameters affecting CE-CL separation and detection, such as reductant concentration, mixing mode of the analytes with CL reagent, CL reaction reagent pH and concentration, stability of luminol-hydrogen peroxide mixed solution were optimized. The limits of detection for chromium(III) and chromium(VI) (3σ) were 6×10⁻¹³ mol l⁻¹ (mass concentration 12 zmol) and 8×10⁻¹² mol l⁻¹ (160 zmol), respectively. This method offered potential advantages of simplicity, sensitivity, selectivity and applicability to the determination of Cr(III) and Cr(VI) in environmental water.     

Determination of Cr(VI) in the presence of Cr(III) and humic acid by cathodic stripping voltammetry

A voltammetric procedure in the flow system for determination of traces of Cr(VI) in the presence of Cr(III) and humic acid is presented. The calibration graph is linear from 5×10⁻¹⁰ to 1×10⁻⁷ mol l⁻¹ for an accumulation time of 120 s. The R.S.D. for 1×10⁻⁸ mol l⁻¹ Cr(VI) is 5.3% (n=5). The detection limit estimated from 3σ for a low concentration of Cr(VI) and accumulation time of 120 s is 2×10⁻¹⁰ mol l⁻¹. The method can be used for Cr(VI) determination in the presence of up to 50 mg l⁻¹ of humic acid. The validation of the method was carried out by studying the recovery of Cr(VI) from spiked river water and by the comparison of the results of determination of Cr(VI) in a soil sample. The method cannot be used for analysis of samples containing high concentrations of chloride ions such as seawater and estuarine water.     

Facile preparation of glutathione-stabilized gold nanoclusters for selective determination of chromium (III) and chromium (VI) in environmental water samples

A novel method for selective determination of Cr(III) and Cr(VI) in environmental water samples was developed based on target-induced fluorescence quenching of glutathione-stabilized gold nanoclusters (GSH-Au NCs). Fluorescent GSH-Au NCs were synthesized by a one-step approach employing GSH as reducing/protecting reagent. It was found that Cr(III) and Cr(VI) showed pH-dependent fluorescence quenching capabilities for GSH-Au NCs, and thus selective determination of Cr(III) and Cr(VI) could be achieved at different pHs. Addition of EDTA was able to effectively eliminate the interferences from other metal ions, leading to a good selectivity for this method. Under optimized conditions, Cr(III) showed a linear range of 25–3800 μg L⁻¹ and a limit of detection (LOD) of 2.5 μg L⁻¹. The Cr(VI) ion demonstrated a linear range of 5–500 μg L⁻¹ and LOD of 0.5 μg L⁻¹. The run-to-run relative standard deviations (n = 5) for Cr(III) and Cr(VI) were 3.9% and 2.8%, respectively. The recoveries of Cr(III) and Cr(VI) in environmental water samples were also satisfactory (76.3–116%). This method, with its simplicity, low cost, high selectivity and sensitivity, could be used as a promising tool for chromium analysis in environmental water samples.     

Single-drop micro-extraction and diffuse reflectance Fourier transform infrared spectroscopic determination of chromium in biological fluids

The present paper deals with a new micro-extraction procedure for selective separation of Cr(VI) in the form of a metaloxy anionic species namely dichromate (Cr₂O₇²⁻) with N¹-hydroxy-N¹,N²-diphenylbenzamidine (HOA) in to dichloromethane and its subsequent and rapid diffuse reflectance Fourier transform infrared spectroscopic (DRS-FTIR) determination employing potassium bromide matrix. The diffuse reflectance Fourier transform infrared spectroscopy gives both qualitative and quantitative information about the dichromate. The determination of chromium is based on the analytical peak selection, among the various vibrational peaks, at 902 cm⁻¹. The micro-extraction was based on the liquid-liquid solvent extraction (LLSE) principle. The dichromate binds with the nitrogen and oxygen atoms of N¹-hydroxy-N¹,N²-diphenylbenzamidine (HOA) and forms 1:2, Cr(VI):HOA complex in 0.1 mol L⁻¹ HCl medium. The formation of above complex, in the acidic medium, is confirmed by the appearance of chocolate-brown color in the micro-extract. The speciation studies of Cr(III) and Cr(VI) is done by conversion of Cr(III) into Cr(VI) utilizing H₂O₂ as an oxidizing agent. The chemistry of pure dichromate and that of its HOA complex is discussed. The limit of detection (LoD) and the limit of quantification (LoQ) of the method are found to be 0.01 μg g⁻¹ Cr₂O₇²⁻ and 0.05 μg g⁻¹ Cr₂O₇²⁻, respectively. The standard deviation value and the relative standard value at a level of 10 μg Cr₂O₇²⁻/0.1 g KBr for n = 10 is found to be 0.26 μg Cr₂O₇²⁻ and 2.6%, respectively. The relative standard deviation (n = 8 and 6) for the determination of dichromate (Cr₂O₇²⁻) in real human biological fluid samples is observed to be in the range 3.1-7.8%.     

Simultaneous determination of N-acetyl-p-aminophenol and p-aminophenol with poly(3,4-ethylenedioxythiophene) modified glassy carbon electrode

A sensitive and selective method was developed for the determination of N-acetyl-p-aminophenol (APAP) and p-aminophenol (PAP) using poly(3,4-ethylenedioxythiophene) (PEDOT)-modified glassy carbon electrode (GCE). Cyclic voltammetry and differential pulse voltammetry were used to investigate the electrochemical reaction of APAP and PAP at the modified electrode. Both APAP and PAP showed quasireversible redox reactions with formal potentials of 367 mV and 101 mV (vs. Ag/AgCl), respectively, in phosphate buffer solution of pH 7.0. The significant peak potential difference (266 mV) between APAP and PAP enabled the simultaneous determination both species based on differential pulse voltammetry. The voltammetric responses gave linear ranges of 1.0 × 10⁻⁶-1.0 × 10⁻⁴ mol L⁻¹ and 4.0 × 10⁻⁶-3.2 × 10⁻⁴ mol L⁻¹, with detection limits of 4.0 × 10⁻⁷ mol L⁻¹ and 1.2 × 10⁻⁶ mol L⁻¹ for APAP and PAP, respectively. The method was successfully applied for the determination of APAP and PAP in pharmaceutical formulations and biological samples.     

Analysis of interferents by means a D-optimal screening design and calibration using partial least squares regression in the spectrophotometric determination of Cr(VI)

Using a central composite design, the signal of the process for the spectrophotometric determination of hexavalent chromium (λ = 543 nm) is maximised and its variability minimised using as complexing agent 1,5-diphenylcarbazide in sufficiently acid medium. To analyse the interference of various analytes (Mo(VI), V(V), Fe(III) and Mn(VII)) on the Cr(VI) as a function of concentration of interferent, a factorial design was prepared at three levels of each (zero, medium and high concentration), which implies performing 81 determinations. However, a D-optimal design with just nine experiments is sufficiently good to estimate the model proposed.The interference of these metals makes it impossible to determine Cr(VI) when they are present in the sample. To avoid prior separation steps, a multivariate regression by partial least squares, PLS, is proposed to calibrate the Cr(VI) in the presence of these analytes varying the concentration of the Cr(VI) between 0.1 and 0.9 μg ml⁻¹ and that of the interferents between 3 and 5 μg ml⁻¹. The average errors obtained were 4.5% and 3.29% fitted and in prediction, respectively, with a standard error in prediction (RMSEP) of 0.016% presenting absence of both constant and proportional bias.The detection limit with the PLS regression in the presence of interferents is 0.1 μg ml⁻¹ with a probability of false positive equal to 5% and less than 5% for false negative. The capability of detection is similar to that obtained with the univariate calibration (absorbance at 543 nm) in absence of interferents.With the PLS regression it is possible to discriminate 0.085 μg ml⁻¹ of Cr(VI) in a sample with 0.5 μg ml⁻¹ of Cr(VI) with probabilities of false compliance and false non-compliance equal to 0.05. For the univariate calibration without interferents, it was established at 0.0971 μg ml⁻¹ of Cr(VI) for the same nominal concentration.In relation to interference of V(V), Fe(III) and Mn(VII), the PLS calibration could be an efficient alternative to the separation step for Cr(VI) spectrophotometric determination using 1,5-diphenylcarbazide.     

Silica gel-immobilized-vanillin derivatives as selective solid-phase extractants for determination of chromium (III) in environmental samples by ICP-OES

A method is described for the selective extraction of chromium(III) from aqueous solutions and natural water samples, based on the use of two newly synthesized solid-phase extractors via silica gel-immobilized-vanillin derivatives (I,II). Experimental conditions for effective adsorption of trace levels of Cr(III) were optimized with respect to different experimental parameters using batch and column procedures in detail. The optimum pH values for the separation of Cr(III) simultaneously on the newly sorbents were both 4.0 and complete elution of Cr(III) from the sorbents surface was carried out using 2.0 mL of 0.5 mol L^− 1 HCl. The sorption capacity of phase I towards Cr(III) was found to be 0.700 mmol g^− 1 where the sorption capacity of phase II was 0.538 mmol g^− 1. The detection limits (3σ) of the method defined by IUPAC were found to be 0.87 and 0.64 ng mL^− 1 with enrichment factors of 100 and 75 for phases I and II, respectively. The method has been applied for the determination of Cr(III) in biological materials and water samples with satisfactory results.     

Spectrofluorometric determination and chemical speciation of trace concentrations of chromium (III & VI) species in water using the ion pairing reagent tetraphenyl-phosphonium bromide

A highly selective, and low cost extractive spectrofluorometric method has been developed for determination of trace concentrations of chromium (III & VI) in water samples using the fluorescent reagent tetraphenylphosphonium bromide (TPP⁺·Br⁻). The method was based upon solvent extraction of the produced ion associate [TPP⁺·CrO₃Cl⁻] of TPP⁺·Br⁻ and halochromate in aqueous HCl and measuring the fluorescence quenching of TPP⁺·Br⁻ in chloroform at λ_ex/em = 242/305 nm. The fluorescence intensity of TPP⁺Br⁻ decreased linearly on increasing the chromium (VI) concentration in the range of 1-114 μg L⁻¹. The limits of detection (LOD) and quantification (LOQ) of chromium (VI) were 0.43 and 1.42 μg L⁻¹, respectively. Chromium (III) species after oxidation to chromium (VI) with H₂O₂ in alkaline solution were also determined. Chemical speciation of chromium (III & VI) species at trace levels was achieved. The method was applied for analysis of chromium in certified reference material (IAEA Soil-7) and in tap- and wastewater samples and compared successfully (>95%) with the inductively coupled plasma-mass spectrometry (ICP-MS) results.     

A novel fiber-packed column for on-line preconcentration and speciation analysis of chromium in drinking water with flame atomic absorption spectrometry

A novel on-line preconcentration and determination system based on a fiber-packed column was developed for speciation analysis of Cr in drinking water samples prior to its determination by flame atomic absorption spectrometry (FAAS). All variables involved in the development of the preconcentration method including, pH, eluent type, sample and eluent flow rates, interfering effects, etc., were studied in order to achieve the best analytical performance. A preconcentration factor of 32 was obtained for Cr(VI) and Cr(III). The levels of Cr(III) species were calculated by difference of total Cr and Cr(VI) levels. Total Cr was determined after oxidation of Cr(III) to Cr(VI) with hydrogen peroxide. The calibration graph was linear with a correlation coefficient of 0.999 at levels near the detection limit and up to at least 50 μg L⁻¹. The relative standard deviation (R.S.D.) was 4.3% (C = 5 μg L⁻¹ Cr(VI), n = 10, sample volume = 25 mL). The limit of detection (LOD) for both Cr(III) and Cr(VI) species was 0.3 μg L⁻¹. Verification of the accuracy was carried out by the analysis of a standard reference material (NIST SRM 1643e “Trace elements in natural water”). The method was successfully applied to the determination of Cr(III) and Cr(VI) species in drinking water samples.     

Evidence of Cr(VI) formation during analysis of leather Proposal of an alternative method of analysis through the ion-chromatographic approach and post-column reaction

The formation of Cr(VI) in Cr(III) tanned leather, in neutral and alkaline solution, has been demonstrated by means of crossed experiments using different pH buffers, ethylenediaminetetraacetic acid as Cr(III) complexing agent and NaCl solutions. According to the found results the composition of the extracting solution suitable to extract Cr(VI) amount present in leather was pH 4.4 (which is also the tanned leather natural pH) and 5% NaCl (w/v). Interferences coming from coloured compounds have been eliminated with suitable SPE cartridges. A new protocol for the analysis of Cr(VI) based on ion chromatography and a diphenylcarbazide post-column reaction has been implemented. The use of a large volume injection loop (500 μl) allowed to obtain a very low quantification limit (0.15 mg kg⁻¹) despite the low amount of leather extracted (0.2 g with respect to 2.0 g used by the IUC 18 official method). Evidence of the transient nature of Cr(VI) in leather requires using the external calibration procedure for the correct quantification of the species.     

Flow injection direct spectrophotometric assay for the speciation of trace chromium(III) and chromium(VI) using chromotropic acid as chromogenic reagent

A new rapid and sensitive FI assay is reported for the simultaneous direct spectrophotometric determination of trace Cr(VI) and Cr(III) in real samples. The method is based upon the reaction of Cr(VI) with chromotropic acid (CA) in highly acidic medium to form a water-soluble complex (λ_max = 370 nm). Cr(III) reacts with CA only after its on-line oxidation to Cr(VI) by alkaline KIO₄. The determination of each chromium species in the sample was achieved by absorbance differences. The calibration curves were linear over the range 3-4000 μg l⁻¹ and 30-1200 μg l⁻¹ for Cr(VI) and Cr(III), respectively, while the precision close to the quantitation limit was satisfactory in both cases (s_r = 3.0% for Cr(VI) and 4.0% for Cr(III) (n = 10) at 10 and 50 μg l⁻¹ level, respectively). The method developed proved to be adequately selective and sensitive (c_L = 1 and 10 μg l⁻¹ for Cr(VI) and Cr(III), respectively). The application of the method to the analysis of water samples (tap and mineral water) gave accurate results based on recovery studies (93-106%). Analytical results of real sample analysis were in good agreement with certified values.     

Sensitive and selective spectrofluorimetric determination of chromium(VI) in water by fluorescence enhancement

A new fluorogenic method for the selective and sensitive determination of chromium(VI) in acidic water using rhodamine B hydrazide was developed. This method was based on the oxidation of non-fluorescent rhodamine B hydrazide by potassium dichromate in acidic aqueous conditions to give rhodamine B, which was highly fluorescent, as a product. With the optimum condition described, the fluorescence enhancement at 585 nm was linearly related to the concentration of chromium(VI) in the range of 5.0 × 10⁻⁸ to 2.0 × 10⁻⁶ mol L⁻¹ (2.60-104 ng mL⁻¹) with a correlation coefficient of R² = 0.9993 (n = 18) and a detection limit of 5.5 × 10⁻⁹ mol L⁻¹ (0.29 ng mL⁻¹). The R.S.D. was 2.2% (n = 5). The proposed method was also applied to the determination of chromium(VI) in drinking water, river water and synthetic samples.     

Sensitive assay for oxygen solubility in molten alkali metal carbonates by indirect flame atomic absorption spectrometric Cr(VI) determination

A precise and accurate indirect analytical method for the assessment of O₂ solubility in molten (Li_0.62K_0.38)₂CO₃ and (Li_0.52Na_0.48)₂CO₃ is described. The method is based on the oxidation of Cr₂(SO₄)₃ (added in excess to the melt) by the oxygenate species, which are formed inside the melt when it is in contact with oxygen gas, and subsequent determination of trace amount of Cr(VI) in withdrawn frozen melts by flame atomic absorption spectrometry (FAAS). The samples (1.0-2.0 g) are dissolved in dilute hydrochloric acid at room temperature. The speciation of Cr(VI) is carried out by complexation with ammonium pyrrolidine dithiocarbamate (APDC), followed by extraction into methyl isobutyl ketone (MIBK), which is introduced directly into the flame. Optimisation of the flame composition provided maximum Cr signal in organic phase under lean acetylene-air flame. The separation and preconcentration parameters such as sample volume/extractant volume ratio, pH sample solution, chelating concentration and extraction time are evaluated. Under the optimised conditions Cr(VI) is efficiently separated from Cr(III), which exceeded 200-folds. The results of the analysis of synthetic samples using standards in MIBK medium give rise to recoveries of 98-99%. The Cr(VI) detection limit of 4×10⁻⁶ g L⁻¹ using 12.5-fold preconcentration and relative standard deviation of 1% at the 0.10 mg L⁻¹ level are obtained. The sampling-to-sampling reproducibility was typically 3-5% relative standard deviation. By changing the preconcentration factor and the dilution of the sample melt, it is possible to analyse oxygen concentrations in molten alkali metal carbonates as low as 1.5×10⁻⁹ mol O₂ per gram melt.